I have used the bivvy bag for over a year and it is fine. Light at 366grams, breathable and good waterproofing. I found a site once that was dedicated to bivvy bags (something like bivvybags.co.uk) which reckoned they are a top buy. Anyhow, I'm happy with mine so I thought I'd post the info for others to get a chance to get one at a reasonable price.
Yes tkmaxx seem to have erratic stock levels! Can't wait to try out the hammock I bought there yesterday